(1) Field of the Invention
The present invention relates to piezoelectric/electrostrictive film type elements of a unimorph type or a bimorph to be used as various transducers, various actuators, etc. In particularly, the invention is aimed at advantageously improving durability of such piezoelectric/electrostrictive film type elements without deteriorating their piezoelectric/electrostrictive performance.
The piezoelectric/electrostrictive film type elements according to the present invention are intended to include elements for converting electric energies to mechanical energies, i.e., mechanical displacements, stress or vibration as well as elements for effecting reverse conversions thereof. Since the elements according to the present invention have dielectric property as well as the piezoelectric/electrostrictive performance, they can be also used as filmy condenser elements, etc.
(2) Related Art Statement
The piezoelectric/electrostrictive elements are used in wide fields including various transducers for converting electric energies to mechanical energies, i.e., mechanical displacements, forces or vibration, as well as elements for effecting reverse conversions thereof, various actuators, functional parts operating at a predetermined frequency range such as filters, various display devices such as displays, sound-emitting members such as loudspeakers, sensors such as microphones and ultrasonic wave sensors, etc.
For example, there is known a piezoelectric/electrostrictive element as shown in FIG. 1(a) which comprises a ceramic substrate 1 functioning as a vibrating plate, and a film-type piezoelectric/electrostrictive operating section 5, provided on the substrate 1, that is constituted by a first electrode film 2 as a lower electrode, a piezoelectric/electrostrictive layer 3 and a second electrode film 4 as an upper electrode (JP-A-3-128,681). In addition, there is also known a piezoelectric/electrostrictive element as shown in FIG. 1(b) in which a ceramic substrate 1 is provided with a cavity, and a bottom portion of the cavity is used as a vibrating section 1a, and a piezoelectric/electrostictive operating section 5 is integrally formed on the outer surface of the vibrating section 1a (JP-A-5-49,270).
A ceramic substrates constituting such piezoelectric/electrostictive elements, ceramic materials composed mainly of zirconium oxide partially stabilized with yttrium oxide are generally known (for example, JP-A-5-29,675, JP-A-5-97,437 and JP-A-5-270,912).
Use environments have recently variously changed with diversification in the piezoelectric/electrostrictive devices. Particularly, when the use environment in which the above-mentioned piezoelectric/electrostrictive film type elements are used is of a higher temperature and more humid atmosphere compared with conventional conditions, deterioration in material of the substrate poses a problem.
That is, if the piezoelectric/electrostictive film element is used in the above higher temperature and more humid environment, there is a tendency that the substrate begins to be degraded in the piezoelectric/electrostrictive film type element than the piezoelectric/electrostrictive operating section.
The present invention has been developed in view of the above actual circumstances, and is aimed at the provision of integrated piezoelectric/electrostrictive film type elements having excellent durability and an advantageous producing process thereof, without causing degradation in material of the substrate or lowering the piezoelectric/electrostrictive performance or deteriorating the characteristics of the piezoelectric/electrostrictive layer, even if used under the high-temperature and highly humid atmosphere.
In the following, the elucidation process of the present invention will be explained.
After having repeatedly performed strenuous investigations to accomplish the above object, the present inventors discovered that the substrate was deteriorated by lead element the substrate during the production. That is, although a ceramic material composed mainly of zirconium oxide has been formerly used as the substrate, it was clarified that the lead element entering such a ceramic substrate rapidly damaged the durability of the substrate when used in the high-temperature and highly humid condition.
Therefore, the present inventor then examined causes why the lead element entered into the substrate, and discovered that such a lead element entered the substrate when firing (thermally treating) the piezoelectric/electrostrictive material. That is, a lead-containing materials having excellent properties (such as lead zirconate titanate, etc.) was generally used as the piezoelectric/electrostrictive material, and a film of such a material was ordinarily formed as a piezoelectric/electrostrictive layer, and integrated by firing.
When a piezoelectric/electrostrictive layer made of such a lead-containing material is to be formed (by firing), the atmosphere is formerly controlled under condition with high lead concentration to prevent the evaporation of the lead element contained in the piezoelectric/electrostrictive material during firing, since such evaporation may cause changes in the composition of the piezoelectric/electrostrictive layer to thereby deteriorate the piezoelectric/electrostrictive performance.
As mentioned above, the piezoelectric/electrostrictive layer is formerly fired with a lead concentration-increased atmosphere in the presence of an evaporating source of a lead-containing material or the like so as to prevent the deterioration in the above characteristics. However, when the piezoelectric/electrostrictive layer is fired in the atmosphere with such a high lead concentrated, the lead element in the atmosphere penetrates into the ceramic substrate composed mainly of zirconium oxide, which causes deterioration in the quality of the substrate as mentioned before.
In order to solve the above problems, the present inventors repeatedly performed numerous experiments and examinations, and acquired the following knowledge.
(1) The firing atmosphere needs not always be an atmosphere having a high concentration of lead, even if the piezoelectric/electrostrictive layer contains the lead element. If some lead element evaporates from the piezoelectric/electrostrictive layer during firing, no deterioration occurs in the piezoelectric/electrostrictive layer.
(2) The rate of a heterophase occurring at a surface of the piezoelectric/electrostrictive layer during firing is preferable as an index for the judgment of the evaporated amount of the lead element from the piezoelectric/electrostrictive layer.
(3) The integrated piezoelectric/electrostrictive film type element in which the area rate of the heterophase occurring during firing is controlled to a range of 0.1 to 30% by adjusting the firing conditions suffers from neither degradation in quality in the substrate and deterioration in the characteristics of the piezoelectric/electrostrictive layer even when in use under high-temperature and highly humid atmosphere.
The present invention is based on the above recognition.
That is, the substantial features of the present invention are as follows.
(1) An integrated piezoelectric/electrostrictive film type element having excellent durability and comprising a substrate made of a ceramic material composed mainly of completely stabilized or partially stabilized zirconium oxide, and a piezoelectric/electrostrictive operating section integrated onto the ceramic substrate by a film-forming method, said piezoelectric/electrostrictive operating section comprising a lower electrode, a piezoelectric/electrostrictive layer of a lead element-containing composition, and an upper electrode, wherein a heterophase-occurrence rate at a surface of the piezoelectric/electrostrictive layer is controlled to a range of 0.1 to 30%.
(2) The integrated piezoelectric/electrostrictive film type element of above (1) having excellent durability in which the ceramic substrate is shaped in the form of a thin diaphragm portion, and said piezoelectric/electrostrictive operating section is integrally formed on an outer surface of the diaphragm portion.
(3) The integrated piezoelectric/electrostrictive film type element of above (1) or (2) having excellent durability in which the heterphase-occurence rate is controlled to a range of 1 to 10%.
(4) The integrated piezoelectric/electrostrictive film type element of above (1) or (2) having excellent durability in which the average grain size of crystals constituting the ceramic substrate is 0.1 to 2.0 xcexcm.
(5) The integrated piezoelectric/electrostrictive film type element of above (1) or (2) having excellent durability in which a thickness of the piezoelectric/electrostrictive layer is not more than 100 xcexcm.
(6) The integrated piezoelectric/electrostrictive film type element of above (1) or (2) having excellent durability in which a thickness of the piezoelectric/electrostrictive operating section is not more than 150 xcexcm.
(7) The integrated piezoelectric/electrostrictive film type element of above (2) in which a thickness of the diaphragm portion is not more than 50 xcexcm.
(8) A process for producing an integrated piezoelectric/electrostrictive film type element having excellent durability, comprising the steps of preparing a substrate made of a ceramic material composed mainly of completely stabilized or partially stabilized zirconium oxide, and successively forming a lower electrode, a piezoelectric/electrostrictive layer of a lead element-containing composition, and an upper electrode on the ceramic substrate by a film-forming method, said lower electrode, said piezoelectric/electrostrictive layer and the upper electrode constituting a piezoelectric/electrostrictive operating section and at least said piezoelectric/electrostrictive layer being fired, wherein a produced rate of heterophase occurring at a surface of the piezoelectric/electrostrictive layer is controlled to a range of 0.1 to 30% in terms of an area rate by adjusting a concentration of lead in a firing atmosphere and/or an amount and a speed of a running fluid of the firing atmosphere.
(9) The highly durable, integrated piezoelectric/electrostrictive film type element-producing process of above (8) in which the atmosphere is controlled by adjusting at least one condition selected from the following four conditions i) a composition ratio, a configuration, a weight and an arranged location of an evaporating source containing lead as a constituent element; ii) an arranged location of a piezoelectric/electrostrictive material in a firing furnace or a firing vessel; iii) an opening degree of the firing furnace or the firing vessel; and iv) feeding an absorbent for absorbing a lead element in the firing atmosphere.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes could be made by the skilled person in the art to which the invention pertains.